The present invention relates to a filter circuit and, more particularly, to an active bandpass filter circuit capable of controlling independently the quality factor (Q) and center frequency of the filter circuit.
A graphic equalizer or a tone controller generally employed in audio systems includes a bandpass filter circuit for controlling the response of a speaker or listening room according to music being played. The filter circuit is usually arranged to independently adjust the center frequency and the quality factor thereof. It is preferable that such filter circuit can easily and independently adjust the center frequency and quality factor (Q value) thereof, receive music signals, whose dynamic range or allowable amplitude range is wide, without generating distortion, and have a simplified circuit configuration.
A prior art bandpass filter circuit shown in FIG. 1 comprises a band elimination filter circuit 10 and a subtraction circuit 30. The band elimination filter circuit 10 includes a twin-T network 11 having variable resistors 12-14 and capacitors 15-17 configured as shown in FIG. 1, first and second buffer amplifiers 18, 19 each having a gain of unity, and a potentiometer 20. The subtraction circuit 30, which serves to subtractively combine input and output signals of band elimination filter circuit 10 to provide bandpass characteristic, includes an operational amplifier 31 and resistors 32 to 35. Variable resistors 12, 13 and 14 are ganged with one another and adapted to adjust the center frequency of a pass band of filter circuit. The potentiometer 20 is adapted to adjust the quality factor (Q).
For the purpose of obtaining desired frequency responses by this prior art bandpass filter circuit, resistors 12 to 14 and capacitors 15 to 17 must be under a severe relation to have such values, respectively, as shown in FIG. 1. It is also very difficult to adjust values of variable resistors 12, 13 and 14 in an accurately associated manner to change the center frequency. Gang error is caused because of mismatching of variable resistors 12, 13 and 14, thereby deteriorating the bandpass characteristic. Gang error increases exponentially with the increased number of elements to be ganged.
FIG. 2 shows another prior art band elimination filter circuit 10 which may be used as the band elimination filter of the bandpass filter circuit shown in FIG. 1 which includes an operational amplifier 21, a buffer amplifier 22 having a gain of unity, resistors 23, 24, a potentiometer 25, a series circuit 36 of variable resistor 37 and capacitor 38, and a parallel circuit 39 of variable resistor 40 and capacitor 41. Variable resistors 37 and 40 are ganged with each other and adapted to adjust the center frequency of the bandpass filter. The potentiometer 25 is adapted to adjust the quality factor.
In the case of filter shown in FIG. 2, only two variable resistors may be ganged with each other to adjust the center frequency, so that gang error is reduced to some extent as compared with the circuit of FIG. 1. However, the circuit of FIG. 2 has such a drawback that the dynamic range of input signals is limited lower than half the maximum output voltage of amplifier 21 because the gain of operational amplifier 21 (two in this case) is determined by the ratio of resistors 23 and 24.
Both the bandpass filter circuit of FIG. 1 and the one in which the band elimination circuit of FIG. 2 is employed are complicated in circuit configuration to thereby increase cost.